Field
The present disclosure relates to telecommunications apparatus and methods.
Description of Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
The present disclosure relates to wireless telecommunications systems and methods, and in particular to systems and methods for restricted frequency resource/virtual carrier operation in wireless telecommunication systems.
Mobile communication systems have evolved over the past ten years or so from the GSM System (Global System for Mobile communications) to the 3G system and now include packet data communications as well as circuit switched communications. The third generation partnership project (3GPP) is developing a fourth generation mobile communication system referred to as Long Term Evolution (LTE) in which a core network part has been evolved to form a more simplified architecture based on a merging of components of earlier mobile radio network architectures and a radio access interface which is based on Orthogonal Frequency Division Multiplexing (OFDM) on the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) on the uplink.
The deployment of third and fourth generation networks has led to the parallel development of a class of devices and applications which, rather than taking advantage of the high data rates available, instead take advantage of the robust radio interface and increasing ubiquity of the coverage area. Examples include so-called machine type communication (MTC) applications, some of which are in some respects typified by semi-autonomous or autonomous wireless communication devices (MTC devices) communicating small amounts of data on a relatively infrequent basis. Examples include so-called smart meters which, for example, are located in a customer's home and periodically transmit data back to a central MTC server relating to the customer's consumption of a utility such as gas, water, electricity and so on. Smart metering is merely one example of potential MTC device applications. Further information on characteristics of MTC-type devices can be found, for example, in the corresponding standards, such as ETSI TS 122 368 V11.6.0 (2012-09)/3GPP TS 22.368 version 11.6.0 Release 11) [1].
Whilst it can be convenient for a terminal such as an MTC-type terminal to take advantage of the wide coverage area provided by a third or fourth generation mobile telecommunication network there are at present disadvantages. Unlike a conventional third or fourth generation mobile terminal such as a smartphone, a primary driver for MTC-type terminals will be a desire for such terminals to be relatively simple and inexpensive. The type of functions typically performed by an MTC-type terminal (e.g. simple collection and reporting/reception of relatively small amounts of data) do not require particularly complex processing to perform, for example, compared to a smartphone supporting video streaming. However, third and fourth generation mobile telecommunication networks typically employ advanced data modulation techniques and support wide bandwidth usage on the radio interface which can require more complex and expensive radio transceivers and decoders to implement. It is usually justified to include such complex elements in a smartphone as a smartphone will typically require a powerful processor to perform typical smartphone type functions. However, as indicated above, there is now a desire to use relatively inexpensive and less complex devices which are nonetheless able to communicate using LTE-type networks.
With this in mind there has been proposed a concept of so-called “virtual carriers” operating within the bandwidth of a “host carrier”, for example, as described in GB 2 487 906 [2], GB 2 487 908 [3], GB 2 487 780 [4], GB 2 488 513 [5], GB 2 487 757 [6], GB 2 487 909 [7], GB 2 487 907 [8] and GB 2 487 782 [9]. One principle underlying the concept of a virtual carrier is that a frequency subregion (subset of frequency resources) within a wider bandwidth (greater range of frequency resources) host carrier is configured for use as a self-contained carrier for at least some types of communications with certain types of terminal device. For example, a terminal device may be configured to receive at least some communications from the base station within a restricted subset of transmission resources selected from within the system frequency bandwidth whereby the restricted subset of transmission resources comprises a downlink channel having a channel bandwidth which is smaller than the system frequency bandwidth.
In some virtual carrier implementations, such as described in references [2] to [9], all downlink control signalling and user-plane data for terminal devices using the virtual carrier may be conveyed within the subset of frequency resources associated with the virtual carrier. A terminal device operating on the virtual carrier is made aware of the restricted frequency resources and need only receive and decode a corresponding subset of transmission resources to receive data from the base station. One advantage of this approach is to provide a carrier for use by low-capability terminal devices capable of operating over only relatively narrow bandwidths. This allows devices to communicate on LTE-type networks, without requiring the devices to support full bandwidth operation. By reducing the bandwidth of the signal that needs to be decoded, the front end processing requirements (e.g., FFT, channel estimation, subframe buffering etc.) of a device configured to operate on a virtual carrier are reduced since the complexity of these functions is generally related to the bandwidth of the signal that needs to be processed.
Other virtual carrier approaches for reducing the required complexity of devices configured to communicate over LTE-type networks are proposed in GB 2 497 743 [10] and GB 2 497 742 [11]. These documents propose schemes for communicating data between a base station and a reduced-capability terminal device whereby physical-layer control information for the reduced-capability terminal device is transmitted from the base station using subcarriers selected from across a full host carrier frequency band (as for conventional LTE terminal devices). However, higher-layer data for reduced-capability terminal devices (e.g. user-plane data) is transmitted using only subcarriers selected from within a restricted subset of carriers which is smaller than and within the set of subcarriers comprising the system frequency band. Thus, this is an approach in which user-plane data for a particular terminal device may be restricted to a subset of frequency resources (i.e. a virtual carrier supported within the transmission resources of a host carrier), whereas control signalling is communicated using the full bandwidth of the host carrier. The terminal device is made aware of the restricted frequency resource, and as such need only buffer and process data within this frequency resource during periods when higher-layer data is being transmitted. The terminal device buffers and processes the full system frequency band during periods when physical-layer control information is being transmitted. Thus, the reduced-capability terminal device may be incorporated in a network in which physical-layer control information is transmitted over a wide frequency range, but only needs to have sufficient memory and processing capacity to process a smaller range of frequency resources for the higher-layer data. This approach may sometimes be referred to as a “T-shaped” allocation because the area of the downlink time-frequency resource grid to be used by the reduced-capability terminal device may in some cases comprise a generally T-shape.
Virtual carrier concepts thus allow terminal devices having reduced capabilities, for example in terms of their transceiver bandwidth and/or processing power, to be supported within LTE-type networks. As noted above, this can be useful to allow relatively inexpensive and low complexity devices to communicate using LTE-type networks. However, providing support for reduced capability devices in a wireless telecommunications system which is generally based around existing standards can require additional considerations for some operational aspects of wireless telecommunications systems to allow the reduced-capability terminal devices to operate in conjunction with conventional terminal devices.
One area where the inventors have recognised a need for new procedures concerns the reporting on radio channel conditions in wireless telecommunications systems supporting virtual carrier operations.
Wireless telecommunications systems can allow for so-called link adaptation by a network scheduling entity. Link adaptation allows a base station to modify its transmissions characteristics in a manner which takes account of channel conditions existing between the base station and a terminal device based on channel state information received from the terminal device. For example, higher data rates may be used when channel conditions are good compared to when channel conditions are bad. A significant aspect of link adaptation in LTE-based networks is channel quality indicator (CQI) reporting. As is well established, a terminal device may measure the channel quality of a downlink communication and report it back to the base station as a CQI report. The base station may then perform link adaptation based on the CQI report.
Existing LTE standards provide for CQI reports with two types of bandwidth. One is known as wideband CQI and the other is known as subband CQI. For wideband CQI a single CQI value is established for a carrier's full bandwidth and reported to the base station. For subband CQI, the full bandwidth is in effect split into more than one subband, and a CQI value is established for each subband. The wideband CQI approach is simple and provides for compact signalling whereas the subband CQI approach can allow a scheduler to take account of frequency selective channel conditions (e.g. frequency-dependent fading).
The inventors have recognised that particular considerations can apply when considering channel conditions, for example through CQI measurement and reporting, in the context of virtual carriers and there is therefore a desire to provide for improved schemes for reporting on channel conditions in wireless telecommunications systems supporting virtual carrier operation.